The present invention relates to variable speed controls for direct current electric motors; and more particularly to such controls for operating hand-held, battery powered tools which are driven by an electric motor.
Hand-held power tools, such as electric drills and dry-wall screwdrivers, utilize a DC electric motor to rotate a bit which either drills a hole or turns a screw. These power tools often have a pistol-like grip with a trigger which is manually operated by the user of the tool with the speed of the motor being controlled by the degree to which the user presses the trigger. This allows the speed of the drill or screwdriver bit to be varied depending upon the particular application for the tool. For example, the speed of a drill bit can be controlled to correspond to the hardness of the material being drilled; e.g. the harder the material, the slower the drill bit should rotate.
The trigger, which is spring biased into an off position, is mechanically connected to a switch which closes upon the user depressing the trigger from that off position. The trigger also is mechanically connected to a wiper of a potentiometer in the speed control circuit and the resistance of the potentiometer changes with trigger movement. One type of control circuit responds to changes in the potentiometer resistance by pulse width modulating the electric current applied to the motor. That is, the electric current is applied in the form of pulses having duty cycles that vary to control the motor speed. The greater the duty cycle, the longer the current pulse, and the faster the motor operates.
The trigger operates several contacts of the speed control switch and it is desirable to have the switch be compact and cost effective while providing smooth control of the tool's speed.